![]() ![]() With specific reference to population-level handedness, a number of different evolutionary models have been proposed, with each emphasizing the significance of human-specific adaptations that may have selected for increasing left hemisphere control of motor actions, including hand preferences. It has been known for many years that most animals show individual limb preferences (paw or hand preference), 21, 26 but the evolutionary debate has centered around the emergence of population-level behavioral and brain asymmetries. 21- 25 Population-level asymmetry refers to consistent directional asymmetries in a significant proportion of the subjects within a sample. The functional and anatomical association between left-lateralization of the IFG and PT, handedness, and language have led some to suggest that population-level asymmetries in these brain regions are uniquely human, and a consequence of evolutionary selection for increased motor control for language and speech that evolved after the split between humans and chimpanzees approximately 5–6 million years ago. The PT is the flat bank of tissue that lies posterior to Heschl’s gyrus (HG), and numerous studies have shown that there is a leftward asymmetry in approximately 70–75% of human brains, though a smaller proportion of left-lateralized individuals have been reported among left-handed subjects 17- 20. Less controversial is the evidence of leftward neuroanatomical asymmetries in the posterior temporal lobe, specifically the planum temporale (PT). 10- 13 Specifically, in humans, the IFG is morphologically composed of three regions: the pars opercularis, pars triangularis and pars orbitalis, and there is some evidence indicating that this region of the brain is larger in the left than the right hemisphere, particularly among right-handed individuals, though this depends on whether the volume of the entire gyrus is quantified or only the portion composed of gray matter 14- 16 (see below). 5 There is also evidence of neuroanatomical asymmetries in humans for both the inferior frontal gyrus and the posterior temporal lobe, which many believe may underlie the functional specializations found for Broca’s and Wernicke’s areas. 9 Specifically, the left inferior frontal gyrus (IFG), as well as the left posterior temporal lobe of the human brain, which include Broca’s and Wernicke’s areas, play important roles in the production and comprehension of gestural and spoken language. 1- 3 In addition, clinical, experimental, and functional imaging studies have shown that some, though not all, aspects of language and speech are lateralized to the left hemisphere, 4- 8 particularly among right-handedness individuals. It has been well documented that, though there is some cultural variation, a significant majority of humans self-report preferring to use the right hand for a variety of actions. The two most commonly studied and documented specializations in the human brain are language and handedness. Hemispheric specialization refers to sensory, motor, or cognitive functions that are differentially processed by the left and right cerebral hemispheres. These results stand in contrast to reported claims of significant differences in the distribution of handedness in humans and chimpanzees, and I discuss some possible explanations for the discrepancies in the neuroanatomical and behavioral data.Ī fundamental aspect of the human central nervous system is hemispheric specialization. ![]() Similarly, there is good evidence that chimpanzees show population-level handedness, though, again, the expression of handedness is less robust compared to humans. ![]() When similar methods and landmarks are used to define the PT and IFG, humans and chimpanzees show similar patterns of asymmetry in both cortical regions, though humans show more pronounced directional biases. I also review evidence of population-level handedness in captive and wild chimpanzees. Here, I summarize comparative data on neuroanatomical asymmetries in the planum temporale (PT) and inferior frontal gyrus (IFG) of humans and chimpanzees, regions considered the morphological equivalents to Broca’s and Wernicke’s areas. Recent studies in nonhuman animals, notably primates, have begun to challenge this view. Many historical and contemporary theorists have proposed that population-level behavioral and brain asymmetries are unique to humans and evolved as a consequence of human-specific adaptations such as language, tool manufacture and use, and bipedalism. ![]()
0 Comments
Leave a Reply. |